diff --git a/packages/backend/src/routers/filesystem_api/batch/all.js b/packages/backend/src/routers/filesystem_api/batch/all.js
index b367ed52..5b30d691 100644
--- a/packages/backend/src/routers/filesystem_api/batch/all.js
+++ b/packages/backend/src/routers/filesystem_api/batch/all.js
@@ -27,6 +27,7 @@ const { BatchExecutor } = require("../../../filesystem/batch/BatchExecutor");
 const { TeePromise } = require("../../../util/promise");
 const { EWMA, MovingMode } = require("../../../util/opmath");
 const { get_app } = require('../../../helpers');
+const { valid_file_size } = require("../../../util/validutil");
 
 const commands = require('../../../filesystem/batch/commands.js').commands;
 
@@ -189,9 +190,11 @@ module.exports = eggspress('/batch', {
 
             if ( fieldname === 'fileinfo' ) {
                 const fileinfo = JSON.parse(value);
-                if ( fileinfo.size < 0 ) {
+                const { v: size, ok: size_ok } = valid_file_size(fileinfo.size);
+                if ( ! size_ok ) {
                     throw APIError.create('invalid_file_metadata');
                 }
+                fileinfo.size = size;
                 fileinfos.push(fileinfo);
                 return;
             }
diff --git a/packages/backend/src/routers/filesystem_api/write.js b/packages/backend/src/routers/filesystem_api/write.js
index 6c18ae9d..2bb289b7 100644
--- a/packages/backend/src/routers/filesystem_api/write.js
+++ b/packages/backend/src/routers/filesystem_api/write.js
@@ -26,6 +26,7 @@ const Busboy = require('busboy');
 const { TeePromise } = require('../../util/promise.js');
 const APIError = require('../../api/APIError.js');
 const api_error_handler = require('../../api/api_error_handler.js');
+const { valid_file_size } = require('../../util/validutil.js');
 
 // -----------------------------------------------------------------------//
 // POST /up | /write
@@ -119,9 +120,19 @@ module.exports = eggspress(['/up', '/write'], {
         const {
             filename, mimetype,
         } = details;
+        
+        const { v: size, ok: size_ok } =
+            valid_file_size(req.body.size);
+            
+        if ( ! size_ok ) {
+            p_ready.reject(
+                APIError.create('invalid_file_metadata')
+            );
+            return;
+        }
 
         uploaded_file = {
-            size: req.body.size,
+            size: size,
             name: filename,
             mimetype,
             stream,
diff --git a/packages/backend/src/util/validutil.js b/packages/backend/src/util/validutil.js
new file mode 100644
index 00000000..cdbdc7ac
--- /dev/null
+++ b/packages/backend/src/util/validutil.js
@@ -0,0 +1,14 @@
+const valid_file_size = v => {
+    v =  Number(v);
+    if ( ! Number.isInteger(v) ) {
+        return { ok: false, v };
+    }
+    if ( ! (v >= 0) ) {
+        return { ok: false, v };
+    }
+    return { ok: true, v };
+};
+
+module.exports = {
+    valid_file_size,
+};